Field of the Invention
This invention generally relates to low density cement compositions. More specifically, this invention relates to low density cements for use in high temperature conditions for cementing wells.
Background of the Invention
Cementing a string in one stage is a challenging task, especially in the presence of weak formations, including where there is a lost circulation zone. Cement slurry losses during placement can occur when the equivalent circulating density (ECD) exceeds 10.96 lb/gal during placement. A conventional method to overcome this challenge is to use multi-stage cementing by setting the stage tool above the loss circulation zone. However, stage tools can fail, thus causing serious delay and economic loss due to remedial operations such as perforation and squeeze jobs. Furthermore, complexity of multi-stage installing also leads to failure. In addition, stage tools are considered weak points and not good for use as long term seals. A second method for zonal isolation is to use low density cement.
Low density cement slurries can be used to reduce the hydrostatic pressure on weak formations and to cement lost circulation zones. Examples of low density cements are water extender cements, foam cements, and hollow microsphere cements.
Water extender cements are generally limited in density to about 11.50 lb/gal. Cement fallback often occurs and top of set cement can be hundreds of feet below the ground level because the formations cannot withstand the hydrostatic load exerted by water extender cements even if full circulation is maintained to the surface and cement returns are noted. Sulfide-containing water can also corrode the uncemented casing resulting in expensive remedial treatments. Water extender cements can be used in multistage operations, however multistage cementing shows only limited success, as discussed above.
To avoid multistage tools failure as well as the problems associated with the other types of cements noted above, hollow microsphere cement has been developed. Hollow microsphere cement can be used to prepare cement having a density of about 9.22 lb/gal cement. A gas can be contained in the microspheres to further reduce cement density further down to about 8.02 lb/gal.
There are several methods to prepare hollow microsphere cement. One way is to prepare a mixture of coarse and fine cement particles, fly ash, fumed silica, hollow glass spheres, and water. Another method is to add hollow glass or ceramic microspheres to plasticizer, cement, and a strengthening agent such as aluminum metal powder and sodium sulfate. However, these existing hollow microsphere cement formulations are subject to problems related to fluid loss control and low compressive strength. A new design of low density cement to enhance and maintain the compressive strength that can eliminate multistage cementing operations, when possible, is needed.